Hydrostatic transmission control

ABSTRACT

A hydrostatic transmission of the kind useful in an automotive vehicle is controlled automatically by adjustment of the output of a driven hydraulic generator (i.e., a pump) to maintain the torque absorbed by the generator substantially equal to the vehicle driving motor torque. Such adjustment is effected in accordance with fluid pressure established in accordance with the generator speed but modified in accordance with fluid pressure established between the generator and a receptor (i.e., a hydraulic motor) connected hydraulically thereto and connectable mechanically to one or more traction wheels of the vehicle.

.PATENIE'DMARE] I972 3,650,108

INVENTOR OLIVIER \SAAc B M Maya W ATTORNEY United States Patent IsaacMar. 21, 1972 [54] HYDROSTATIC TRANSMISSION 3,054,263 9/1962 Budzich etal. ..60/53 CONTROL 3,214,911 11/1965 Kempson ..60/53 3,402,549 9/1968Connett et a1. ..60/l9 [72] Inventor: Olivier Isaac, Lyon, France [73]Assignee: Societe lndustrielle Generale de Mecanique Geoghegan AppliqueeS.I.G.M.A., Paris, France y [221 hled: Sept. 17, 1969 1 1 ABSTRACT [21 1APPL Nu: 858,579 A hydrostatic transmission of the kind useful in anautomotive Foreign Application Priority Data vehicle is controlledautomatically by adjustment of the output of a driven hydraulicgenerator (i.e., a pump) to maintain [3 the torque absorbed by thegenerator substantially equal to the vehicle driving motor torque. Suchadjustment is effected Sept. 20, 1968 France ..167081 in accordance withfluid pressure established in accordance with the generator speed butmodified in accordance with [52] US. Cl ..60/53 R, 60/52 VS, 60/19 fluidpressure established between the generator and a weep 1 --F16d31/00 tor(Le, a hydraulic motor) connected hydraulically thereto Field Of Search52 53 A and connectable mechanically to one or more traction wheels I ofthe vehicle.

[56] References Cited 2 Claims, 1 Drawing Figure UNITED STATES PATENTS t2,238,061 4/1941 Kendrick ..6Q /53 ux Wm Z? I t HYDROSTATIC TRANSMISSIONCONTROL This invention relates to control, especially self-regulation,of hydraulic transmissions. lt concerns hydrostatic transmissions of thekind comprising an hydraulic generator or pump driven by a drive motorsuch as a heat engine or electric motor, at least one receptor orhydraulic motor interconnected mechanically to a propulsion deviceliable to be subjected to variable external forces, two conduitsarranged in a closed main circuit between the generator and thereceptor, and a mechanism adapted to vary the output of the generator,i.e., the delivery per shaft revolution thereof. It concerns moreparticularly, but not exclusively, as likely to be the most interestingapplication thereof, those that are installed in automotive vehicles,the propulsion device then being a traction wheel (or a group oftraction wheels) of a vehicle.

lt is known that in this class of hydrostatic transmissions therelationship between the respective speeds of the prime mover or motorshaft and the hydraulic motor or receptor shaft is fixed by the value ofthe generator output when the receptor has a constant output, ordelivery per shaft revolution.

For the given external forces acting upon the propulsion means, it issuitable that the relationship be chosen in such a way that the torquerequired from the motor, when the latter rotates at a given speed, issubstantially equal to the effective torque that the motor can furnishat that speed, in accordance with its characteristic torque-number ofrevolutions per unit time, the torque required from the motor beingequal to the torque absorbed by the generator.

As the external forces change, it is necessary to effect continualadjustment of this relationship in order to maintain the equalitybetween the torque required from the motor and the torque it canfurnish; an inequality causes underspeeding or overspeeding of themotor. Such continual adjustment in relationship thereof is a burdensometask for the driver of the automotive vehicle.

A primary object of the present invention is to remedy thisinconvenience while rendering the adjustment ofthat relationshipautomatic.

Other objects of this invention, together with means and methods forattaining the various objects will be apparent from what followshereinafter.

The invention consists principally, in hydrostatic transmissions of thekind concerned in providing-at the same time as the aforesaid mechanismis controlled by means subject to the pressure of a fluid-an auxiliarycircuit adapted to furnish that fluid under a pressure varying in thesame sense as the generator speed and in a sense opposite to the valueof the external forces, the overall effect being such that, whatever thevalue of the external forces, the torque absorbed by the generator issubstantially equal to the torque that the motor driving it can furnish.

The invention consists, aside from this principal arrangement, incertain other arrangements that are utilized preferably at the same timeand which will be discussed more specifically hereinafter.

This invention will be understandable, in any case, with the aid of thefollowing description as well as the accompanying diagram, relating to apreferred arrangement given purely by way of example.

The single view of this diagram shows schematically a preferredembodiment of a hydrostatic transmission constructed according to theinvention.

This hydrostatic transmission comprises, as a whole, an hydraulicgenerator (or pump) 1 driven by a motor 2, a receptor (or hydraulicmotor) 3 interconnected mechanically by its shaft 44 to a propulsiondevice (made up of a traction wheel, not shown) liable to be subjectedto variable external forces, two conduits 4 and 5 arranged in a closedcircuit between generator 1 and receptor 3, and a mechanism adapted toeffect variation in the generator output, i.e., the delivery perrevolution of drive shaft 7 thereof.

Drive motor 2 can be a Diesel engine or an internal combustion enginehaving a control such as a pedal that the driver can depress todetermine, according to the type of feed associated with motor 2, thetorque furnished by that motor or the speed of drive shaft 7.

Numerous designs for the aforesaid mechanism are known; the generatorcan be, for example, a pump with axial pistons whose strokes have theiramplitudes controlled by the angular position of a cam forming anessential part of that mechanism, the position thereof being determined,for example, by that of a movable rod.

That being so, according to the invention, at the same time as theaforesaid mechanism is controlled by means (hereinafter called firstmeans) responsive to the pressure of a fluid, an auxiliary circuit isprovided that is adapted to furnish such fluid under a pressure varyingin the same sense as the speed of generator 1 and in a sense opposite tothe value of the aforementioned external forces, the torque absorbed bygenerator 1 being substantially equal to the torque that the motor canfurnish.

The aforesaid auxiliary circuit is advantageously made up of a pump 11interconnected mechanically to generator 1 by a shaft 12; this pumptakes in fluid and discharges it into conduit 27 provided with a choke13 having an adjustable opening to a discharge space. Progressiveopening of choke 13 is controlled by means 15 (hereinafter called secondmeans) such as a hydraulic jack responsive to the pressure prevailing inthat one of channels 4 and 5 which joins the intake of the receptor 3 tothe outlet of generator 1, the fluid under pressure in conduit 27 beingwithdrawn ahead of choke 13 and adapted to actuate the aforesaid firstmeans. In the usual case, where the fluid is a liquid, theaforementioned discharge space comprises a reservoir or flume 14 fromwhich pump 11 can be refilled. For clarity of the drawing this reservoirhas been represented at several places therein.

The variable opening of choke 13 can be obtained, for example, by thedisplacement of a pointer (not shown) juxtaposed thereto.

The aforesaid second means is advantageously made up of a piston 16slidably fitting in a cylinder 17, such piston being subjected, in asense corresponding to an increase in the opening of choke 13, to thepressure prevailing in conduit 4 or 5 and, in the opposite sense, to theaction of a retracting device 18 (composed of a spring, for example).

According to a first embodiment, it is assumed first of all that thegenerator is of an irreversible kind, whereupon the direction ofcirculation of fluid between generator 1 and receptor 3 is unchanging(direction indicated by arrows on the diagram). Conduit 5 then joins theintake of receptor 3 permanently to the outlet of generator 1. Thedirection of fluid flow in conduit 27 is similarly indicated by anarrow.

According to this first embodiment, the aforesaid first means is made upof a piston 19 slidably fitting in a cylinder 21, which is connected toconduit 27 ahead of choke 13, that piston being subject to an elasticretracting device, as in the form of spring 23, for example.

In order to inactivate the aforesaid first means it is beneficial tointerpose between conduit 27 and cylinder 21 a distributor 26, adaptedin a first position to isolate cylinder 21 from that conduit byconnection to reservoir 14, and adapted in a second position to connectthat cylinder to conduit 27.

To protect the closed circuit made up of conduits 4 and 5 from excessivepressures that can occur therein, a pressure limiter 28 is providedwhose inlet 29 is connected to conduits 4 and 5 by way of respectivecheck valves 30 and 31 and whose outlets can be connected to conduits 4and 5, respectively, by check valves 33 and 34. Thus, too great apressure in one of conduits 4 and 5 effects an outflow of liquid throughthe opening of limiter 28 and corresponding valve 30 or 31, andeventually a reintroduction of liquid into the other of these conduitsby opening of valve 34 or 33.

So that conduits 4 and 5 may be filled with fluid, whenever fluidcirculating in the closed circuit formed thereby has been drawn off orhas leaked out, a pump 35 is provided interconnected mechanically togenerator 1 or to motor 2 and discharging thereinto through a filter 36,and valves 33 and 34. Pump 35 is protected against excessive pressure bypressure limiter 37 whose intake 38 is connected to outlet 39 of filter36. Outlet 32 of pressure limiter 28 can also be connected to this inlet38. Pump 35 is able to withdraw liquid from reservoir 14 by way of afilter 43, which also can serve pump 11.

Pump 11 is protected against excessive pressure by a pressure limiter 40whose inlet 41 is connected ahead of choke 13 to conduit 27 of theaforesaid auxiliary circuit.

Movement of piston 19 is preferably communicated to rod 9 by way of arod 10 and an hydraulic amplifier or jack 42 filled by pump 35 throughconduit 45.

According to a second embodiment, it is assumed that generator 1 isreversible, the direction of circulation of fluid between generator 1and receptor 3 being either that indicated by the arrows (vehicle movingforward) or in the opposite direction (moving backward).

In such case, mechanism 6 is arranged so that it effects an increase inthe generator output from an initial null value corresponding to theintermediate position of piston rod 9 depicted in the diagram.Displacement of this rod 9 from its initial intermediate position in onedirection or the other produces an increase in generator output intoconduit 4 or into conduit 5,

Moreover, the first aforesaid means is made up of a pair of pistons 19and 20 slidably fitting in a respective pair of aligned cylinders 21 and22, rod 10 having one end (depicted bent at a right angles to the bodyof the rod) located between the two pistons, to which it is connected byelastic return devices comprising springs 23 and 24. Thus, rod 10 can bedisplaced in one direction or the other from an initial intermediateposition (depicted in the diagram) according as the pressure prevailingin conduit 27 ahead of choke 13 also prevails in cylinder 21 or 22,i.e., as the generator discharges into conduit (forward movement) orconduit 4 (backward movement). At the same time, cylinder 17 isconnected to the one of conduits 4 and 5 into which generator 1discharges.

To permit convenient connection of cylinder 21 or cylinder 22 withconduit 27, distributor 26 is designed so that it can occupy not onlytwo positions, as in the previous embodiment, but three positions. Suchpositions are an intermediate neutral position (depicted) in which bothcylinders 21 and 22 are connected to reservoir 14, while being isolatedfrom conduit 27, and two active positions in which one of the twocylinders is connected to conduit 27 ahead of choke 13 and the other isemptied into reservoir 14.

Finally, for convenient linking of cylinder 17 with the appropriate oneof conduits 4 and 5, a distributor 25 is provided that can beinterconnected mechanically to distributor 26.

In view of the foregoing, whichever embodiment may be adopted, ahydrostatic transmission is obtained with variable output that functionsas follows, this discussion being limited to the conditions existingwhen distributor 26 is in one of its active positions (or in its onlyactive position).

At first it is assumed that the vehicle is at rest, with the motoridling, and that the operator wishes to make the vehicle move forward.In the instance of the second aforementioned embodiment, the operatorputs distributor 25 into the position (depicted) wherein cylinder 17 isconnected to conduit 5 and also puts (by raising it) distributor 26(depicted in neutral) into the position wherein cylinder 21 is connectedto conduit 27 of the auxiliary circuit ahead of choke 13 and cylinder 22is discharging.

When the motor is idling, the pressure induced by pump 11 ahead of choke13 is insufficient to cause displacement of piston 19, and generator 1has negligible output.

As soon as the driver accelerates motor 2, by depressing pedal 8, theoutput of pump 11 (driven at a speed equal or proportional to that ofgenerator 1) increases, and the fluid pressure in conduit 27 increases,thereby displacing piston 19 and rod in the direction (viz, downward)corresponding to an increase in the generator output. This output thushas a value increased all the more as the speed of rotation of motor 2is increased.

When the external forces that act upon the wheel driven by receptor 3increase, the pressure of fluid flowing in conduit 5 increases, therebydisplacing piston 16 in a direction (viz, upward) that effects anenlargement of the choke opening. Such enlargement brings about areduction of fluid pressure in conduit 27 (ahead of the choke) and,consequently, a displacement of piston 19 and of rod 10 in a direction(upward) corresponding to reduction of the output of generator 1.

When pedal 8 is completely depressed, motor 2 rotates at maximum speedand can furnish a given torque.

Variations in the external forces acting upon the wheel driven byreceptor 3 then bring about automatically variations in the position ofpiston 19 and of rod 10, so that the torque absorbed by generator 1 issubstantially equal to the aforesaid given torque that the motor 2 canfurnish.

When the external forces are such that motor 2 brakes the vehicle (goingdown slope) the pressures reverse in conduits 4 and 5, in which eventpiston 16 is no longer subjected to fluid pressure because conduit 5lacks appreciable pressure. Under these conditions, fluid discharged bypump 11 into choke 13 determines a pressure ahead of the choke such thatthe generator output has at each instant the appropriate value so thatthe torque absorbed by generator 1 is substantially equal to the torquethat the motor can furnish.

In the case of the aforementioned second embodiment, if the vehicle isput into reverse, distributors 25 and 26 are put into the oppositepositions to those described hereinabove (i.e., with cylinder 17communicating with conduit 4, instead of conduit 5; and with conduit 27communicating with cylinder 22, instead of cylinder 21, which isdischarging instead), whereupon the functioning is analogous to thatdescribed hereinabove.

It goes without saying, and follows moreover from the foregoing, thatthe invention, having been specifically described and illustrated, isnot at all limited to those ways of practicing it, any more than tothose ways of embodying it from various parts. Modifications may be madetherein while retaining the benefits, and without departing from theconcept of, this invention, which itself is defined in the followingclaims.

I claim:

1. In a hydrostatic transmission, a device for controlling the strokevolume of a variable stroke hydraulic pump, supplying a hydraulic motorthrough a closed circuit, said device comprising:

an auxiliary constant stroke pump to replenish the closed circuit withliquid,

an adjustable choke member, mounted in a hydraulic circuit traversed bya flow of liquid dependent on the rotary speed of the variable strokepump, adapted to create a loss of load,

a first hydraulic jack sensitive to said load loss, adapted to controlthe stroke volume of the variable stroke pump,

a second hydraulic jack sensitive to the delivery pressure of thevariable stroke pump, adapted to control the opening of said chokemember so that, when the delivery pressure of the variable stroke pumpincreases, a reduction in the stroke volume of the latter is effected,

and a tachometric constant stroke pump, distinct from said auxiliarypump, driven at a speed proportional to that of the constant strokepump, said tachometric pump feeding in parallel, the adjustable chokemember and the first hydraulic jack, said choke member being connected,downstream, to a reservoir of liquid at relatively nil pressure.

2. Device according to claim 1 comprising a slide valve having an outputconnected by a supply channel to the second hydraulic jack, said slidevalve having means selective as to direction of operation and having twoinputs, each of said input being connected to a respective side of saidclosed hydraulic circuit, the means selective of the direction ofoperation of said slide valve being adapted to occupy the position forwhich the supply channel of said second hydraulic jack is connected tothat side of the closed circuit where the delivery pressure of thevariable stroke pump is exerted.

1. In a hydrostatic transmission, a device for controlling the strokevolume of a variable stroke hydraulic pump, supplying a hydraulic motorthrough a closed circuit, said device comprising: an auxiliary constantstroke pump to replenish the closed circuit with liquid, an adjustablechoke member, mounted in a hydraulic circuit traversed by a flow ofliquid dependent on the rotary speed of the variable stroke pump,adapted to create a loss of load, a first hydraulic jack sensitive tosaid load loss, adapted to control the stroke volume of the variablestroke pump, a second hydraulic jack sensitive to the delivery pressureof the variable stroke pump, adapted to control the opening of saidchoke member so that, when the delivery pressure of the variable strokepump increases, a reduction in the stroke volume of the latter iseffected, and a tachometric constant stroke pump, distinct from saidauxiliary pump, driven at a speed proportional to that of the constantstroke pump, said tachometric pump feeding in parallel, the adjustablechoke member and the first hydraulic jack, said choke member beingconnected, downstream, to a reservoir of liquid at relatively nilpressure.
 2. Device according to claim 1 comprising a slide valve havingan output connected by a supply channel to the second hydraulic jack,said slide valve having means selective as to direction of operation andhaving two inputs, each of said input being connected to a respectiveside of said closed hydraulic circuit, the means selective of thedirection of operation of said slide valve being adapted to occupy theposition for which the supply channel of said second hydraulic jack isconnected to that side of the closed circuit where the delivery pressureof the variable stroke pump is exerted.